Milling or like machine



Mazh 12, 19461e Y .1. T. BOLAS r-:TAL 2,396,488

MILLING OR LIKE MACHINE Filed may 27, 1944 4 sheets-sheet 1 March 12, 1946. J. T. BOLAS ETAL 2,395,488

MILLING 0R LIKE MACHINE Filed May 27, 1944 4 Sheets-Sheet 2 A A nvenZ'rs L E Ik b QQJV: gu/artis ya@ 7M rugrneg Mach l2, 1946. 1 r BOLAS ETAL 2,396,488

MILLING OR LIKE MACHINE Filed May 27, 1944 4 sheets-sheet 3 March 12,1946. n. BOLAS HAL- .2,396,488

MILLING 0R LIKE MAGHINL1 Y .Filed May 27, 1944 4 SheetsSheebA Patented Mar. 12, 1946 UNITED STATE-s PATENT Vorricis John Thomas Bolas" and Raymond William Ed-` wards, Bristol, England,. assignorsv to The. Bristol, Aeroplane, Company, Limitedr Brist'oL. Exlgland',l a British. company Application May 2.171944, SerialNo. 53?;696 IInGreatLBritain May'20, 1343 'Thisinvention concerns milling or.'- li-lieA nia-` chines of the kind in which a cutter is advanced towards vthe work so thatl it penetratesA the workand' is alsomovedv relativelyto the work succes sively in more than: one direction. The" obj ect=off the' invention is to` prov-ide azmachi'ne of this hindi` in Whichl the successive relative movementsbetween the cutterr and the'work are-automatically and! rapidlybrought into being.

According tothe presentinvention atmillinel or like machine of the kind' described comprisesA a uid' motor forV moving the cutter, a stopefor: limiting such relative movement, and a valve meansifor controlling the operationl of:- the uid" motor, the arrangementV being thatthe` engage-A ment ofr said stopzbyv the movement off'the cutter' results in apressure iluid increase Whichzactuatesf` saidl valve means whereby` the uid motor 'pro` duces avrnovementcf the cutter inthev direction successive t that'- whichhasl resulted in the; err-- gagement ofi'said'stop;

Although reference ismadet tof ar movement of: the cutter, it' is to bei understood that'lthis': is al movementrelative to the Work; The cutterfmay.l in certainfi cases be stationary and thek Worlcmayl be moved.

According' to another feature'vof the invention. said` cutter movement"V isrlimit'edby. the engagement off a pattern:l (whose shapefisto be reproduced by the cutter) and a feeler' whichiisftrav.- ersed with respect to the pattern.

Wheref the depth to which the cutter pene-V trates the Work varies over the surface ofthe work said pattern and ieeler may limit'themovementl of.' the cutterrelatively: to the' wor-le in this direction also.

A. specicVV embodiment of? the present invention willA now be described' merely by Way: of example with reference to the accompanying drawings which shewv an applicationofi' the invention to`-a machine for milling the. impeller off a: centrifugal supercharger. In the drawings:

Figure- 1 is: a sectional: elevation of such an impeller, i

.Figure-2 is a view inthe direction of the arrow Iliof Figure, 1,

Figures 3'; and 4 are diagrammaticviews show.- ing the relative disposition ofl the4 cutter and impeller` during: the: iirst` part of: the machining operation,

Figures; 5v and 6 are. similarv viewsA to: Figures: 3 and 4 showing:thezrelationshipxot'the cutterand impeller during the second part of the, machining operation',

Figure' 7 isa.- diagrammatic: representation o a; machine for performing. the rst; part of the machiningpperation, and: Figure 8 is a. diagrammatic, representation; of;v ax machine fory performing; the.V second' part, of the: machining operation. With referenceftoliigures- 1 and;2it. will` be noted that'. the impeller Il)4 comprises. a; front` shroud: ag,` a. back shroud b: and, a plurality of radial blades c;

TheA millingl machine which is; about: ta` heV de:- scribed hasY been: constructed: to,` enablethe: fun nel-like spaces: between. the adjacent; blades crtc: be. machined' from a; forging so thatv thev impeller is:in= one piece. In' order to machine any onezsuch i inter-blade. space ittisnecessary.y toLv performitwol machining-operations'. The rstiofv these iscOnicerned with removingthe metal between. theperip-hervor the impellei and* a; line: approximately coincidental:l with thev inner edge,` of the Y front shroud'. a. Hereinafter' this: partv ofi the interblade spacer. will.: be referred. to as; the outerA pocket. The second` machining'. operation is con cernedfwith producing the triangular inner; nOu-1 tion: of the interebl'ade space-l and. this i is referred to hereinafter asthe inner pocket@ To. produce' the outeri pocket the, impeller is mounted so that; it iscapable of' an oscillating movement, about.. axis d (perpendicular tontha` plane; of the paper in Figure. 4): and` is also ca.- pable. of; av swinging movement: about axise. (Elige ure 3); The cutter, whichzis oi` the end, and: side cutting type; ismaintained-v stationaryv with its long axis in a somewhat. radial direction relative tothe impeller. The, Work is-slowly and4 continuously: fedi towards the, cutter so` that. the cutter graduallyV penetrates; towardsJ thc.-` centre of the impellen thisn-feedmovement istaking place the impeller; is alsooscillating about dA and swinging; about e.. The sequence of operation during machining of the outer pocket is asiellows.

Let it be assumedv thatthe: cutter sin the position shown in Figure 4 and; is` in engagement with the back shroudb.l as is shown in Figuref 31 The impeller is-l oscilla-ted` in an anti-clockwise direction about` axis: d2 so that; they cutter is moved over the face. of the back shroud untilv it reaches the blade c which defines theA pocket; being machined. At this point alimiting stop. is engaged and the oscillating movementI of the impeller is changedA to, a, swinging movement. about axis a so thatthe. cutter ismoved over.l the face ofA bladefc until itis brought-,into engagement with-the front shroud. a; This; condition is; shown in; chandotted lines. in ligure 3f; A. further limitingstop is now engaged and the swinging movement is replaced by an oscillating movement which moves the cutter over the face of the shroud a the oscillating movement taking place in a clockwise direction. This oscillating movement is terminated by the engagement of a limiting stop when the cutter engages the face of the other blade c which defines the pocket being machined. The cutter is moved over the face of this blade by a swinging movement which brings the cutter back to its initial position. The sequence of operations is then repeated.

During this sequence of operations the feed movement previously referred to is continuously proceeding so that the cutter is gradually advanced as it moves over the faces of the shrouds and blades. At the periphery of the impeller, the

diameter of the cutter is such that the Waste metal is entirely removed from the outer pocket. At the inner end of the pocket, however, the increased depth of the impeller makes it necessary for the cutter to take trepanning cuts with the result that a wedge-shaped piece of stock remains in the pocket. This is subsequently removed as is described hereinafter.

The second part of the milling operation is performed by advancing the work towards a milling cutter of the end and side cutting type so that the cutter always lies parallel with the axis of rotation of the impeller (sc-m) whilst the latter is successively moved to perform an oscillating motion about axis f and a reciprocating movement in the direction of arrow g (see Figure 8). As a result of these impeller` movements the cutter, which commences the second mathe circumferential movement of the cutter and I,"

replaces it by an inward radial movement along the surface of blade c until it is returned to point h. Whilstthe impeller is moved so that the cutter traces around the triangular path h, y, i, h relative to the impeller, the latter is also continuously moved towards the cutter, so that the depth of the inner pocket increases during the machining operation.

The base of the wedge-shaped piece of stock which remains in the outer pocket extends to Within the machining range of the cutter which performs the second part of the machining'operation so that as the cutter advances into the inner pocket the wedge-shaped piece of stock is removed.

A milling machine in accordance with the present invention is capable of performing the two machining operations briefly outlined above. Each operation calls for certain variations in the construction and operation of the machines however, anda description of two machines each of which is specially adapted for performing'one such operation now follows.

The machine for performing the first operation is shown in Figure 7. Referring to this flgure: the impeller forging is carried by an oscillating table II which is mounted by a pivot IIa on a. workhead generally indicated at I2. The table II is thus capable of oscillating about axis d whilst the workhead I2 as a whole iS .11101111555 by a pivot I2a on a base |00 so that it is capable of being swung about axis e. The oscillating and swinging motions referred to are those already described with reference to Figures 3 and 4. The cutter I3 is carried by the base |00 and is suitably driven.

In order to oscillate the table II a hydraulic motor I4 is mounted on the workhead I2 and is connected to the table II at one side thereof byirod I5. Similarly in order to swing the head I2 another hydraulic motor I6 is mounted on the base |00 and is connected to the head by a rod I'|.

Therotary cutter I3 is stationary and the infeed movement is performed by advancing the head I2 (and with it the impeller I0) towards the cutter, for which purpose another hydraulic motor (not shown) is provided. This in-feed move ment is continuous in its operation from thc time when the cutter engages the peripheral edge of the impeller until it has completed machining the outer pocket whereupon the in-feed is stopped.

The pressure fluid for operating the reciprocating hydraulic motors, I4 and I6 is supplied by a constant delivery pump I9. The pressure fluid from the pump I8 is directed to the motors I4, I6 through valve assemblies I9, 20 respectively. For each hydraulic motor the associated valve assembly directs the pressure fluid to one end or the other of the cylinder, that is, it controls the direction of movement of the piston in the cylinder. Associated with the valve assemblies I9 and 20 is a further valve assembly 2| which houses a plurality of pilot valves. The pilot valve assembly is suitably connected with the assemblies I9, 20 and the general operation oi l the hydraulic mechanisms is that pressure fluid is supplied by pump I8 selectively to hydraulic motors I4 and I 6 through the valve assemblies I9, 20. When the hydraulic motor to which the pump is supplying pressure fluid has completed` its stroke this piston is prevented from further movement and since the pump I8 continues to deliver pressure fluid to the system the pressure of the system as a whole rises. As a consequence of this pressure increase the pilot valves of the assembly 2| are brought into operation to set the valve assemblies I9, 20 so that they will direct pressure fluid selectively to the hydraulic motors so that one of them performs the cutting stroke successive to that which has produced the pressure increase.

Each valve assembly I 9, 20 comprises a changeover valve 22, 23 and a pair of actuators 24, 25 and. 26, 21 respectively.

The pressure fluid from pump I8 passes to valve assembly I9 by pipe 28 and then by pipes 29 or 30 to the hydraulic motor I4 depending upon the position of the change-over valve 22. Similarly the pressure fluid from the pump I8 passes by pipe 49 to the valve assembly 20 and then by pipes 3| or 32 to the hydraulic motor I6 dependent upon the position of the. change-over valve 23. The pressure fluid is also capable ol' passing from pipe 28 through the valve assembly I9 to either of two pipes 33, 34. These pipes lead respectively to pilot valves 35, 36. Thus when the pressure fluid from pipe 28 passes through valve assembly I9 to pipe 30 and the hydraulic motor I4, it also passes by pipe 33 to the pilot valve 35. Similarly when the pressure fluid flows along pipe 28 to pipe 29 and motor I4 it passes by pipe 34 to the pilot valve 3B.

'I'he arrangement of pressure fluid supplies to the hydraulic motor I6 and pilot valves 38, 40,is similarly provided for in,.connection with. valve assembly 29. Thus when pressurefuid flows by pipe 49 to the hydraulic motor IG by pipe 32 it also passes by pipe 31 to' pilot valve 38', and when it passes by pipey 3l to motor I6 it also passes by pipe 39 to pilotvalve 40;

The actuators 24, 25, 26, and 21 are respectively connected with pilot valve 4u, 38, 35: and 3B by means of pipes 4I, 42, 43 and 44.

The pilot valves each comprise a plunger. 4-5 and a spring 4B which maintains the plunger in its lower position so that the pressure fluid from change-over valves 22 or 23 is prevented from passing to the actuatorsy 24-21.

The operation of the machine isas follows.:

The machining operation is commencedv by starting the iii-feed. so that the workhead I2 and the: impeller forging ID' are` moved towards the stationary cutter I3 whilst the latter isV rotated. When the cutter engages with the periphery of the impeller the machiningoperation described' with reference to Figures 3 and' 4 commences. The disposition of the various parts. of` the machine at this stage is shown in Figure 7. The first cutting operation required is effected by' imparting an anti-clockwise oscillation to the impeller III-by motorV I4 (see Figure 4.) and: this is provided for by passing pressure fluid from pump I8 by pipe 28, valve assembly I9 and pipe 29 to the. motor I4. At the samel time pressure fluid passes by pipe 34 to pilot-valve 36. Meantime in order to maintain the cutter in contact withthe back shroud b (which is being machined)v pressurev uid also passes from pump I8 by pipe 49, valve assembly 20, pipe 3|', to motor I6 to maintain it at the end of its stroke as shown in Figure 7. When the work has been oscillated so that the back shroud b of the outer pocket is completely machined the motor I4 will have completed the limit of its stroke and the piston is prevented from further movement by its engagement with. the cyl,- inder. The pump I8 continuestodeliver. pres'- sure. uid4 to the system however and the fluid pressure accordingly rises. When;A the pressure increase has reached Va predetermined value. pilot valves 3B and 40 will, have their plungers raised against. the pressure of the springs 46 so thatthe pressure uid is enabled to pass by pipes 44. and 4I to the. actuators 21 and 24 respectively. As is clear from Figure 7 thepiston of actuator 24 is in its extreme right-handy position so that the admission of pressure oil by pipe 4I has no eect in the setting ofv the various parts ofthe valve assembly I9. The pressure fluid moving along pipe 44 from pilot valve 35 passes to the actuator 2T and moves the piston of the latte:` to its extreme left-hand position, the actuator 26`being connected to a drain 56 by the pipe 43 and. valve 35. As a consequence the valve 23 is moved from the position shown inr Figure 7 so that it assumes the left-hand position.

The conditions now prevailing are that pressure fluid will continue to pass` by pipe 28, valve assembly I9 andrpipe 29 to the motor I4 so that the latter maintains the first irnpeller bladev associated withthe pocket being machined against the cutter I3. The actuation of the change-over valve of assembly has directed pressure fluid from pipe 49 through the assembly and by pipe 32 to the motor I6 so that the latter swings the head I2 about axis e and thus performs the machining operation on the'surface of the 'rst iinpeller blade (see also Figure 3). This operation continues until the piston of motor I6 reaches the limit of its travel, vwhich occurs when the impeller I (I has been moved to bring the. cutter repeated.

I3 against the surface of the front'- shroudwa. Thepump I3 willv continue to supply'pressure iiuid to the motors I4, and I8 so that the. pressure: of the system rises until aL predetermined value is reached. The pi1ot1valves3I and 38.will there fore beopened since this increased pressurey of the fluid is communicated Vto these' by pipes 3'4 and 31..

The opening of pilot valves 36, 38 enables .the pressureV iiuid tol pass. to actuators 25` and?. 21. As a consequence, the actuator 25 will. be: oper'- atedto move the change-over valve 22 toitslefthand'. positionA whilst actuator 21 which is already in its left-hand position makes no changeinthe valveassembly 2D. Movement of.. valve 2-2.as; de.- scribed enables: pressure fluid from, pump I9' to passi by'pipeI 28, valve assembly I9, and. pipe.; 39 to the motorl I4 which thereupon oscillates the tableV II and impeller IIJ in aclockwise. direction about axis d until the second impellery bladezof the. pocket beingy machined is brought into engagement withthe: cutter. This occurs simule taneously with the piston of motor I4y reaching thelimit of its travel. The pressure ofthe sys- `tem is again built-up and the:r sequence ofopercutter I3 engages the back shroud the pistonl of motor I6 will have reached thelimit of its travel, and as a consequence a pressure increasein the iiuid system will be produced which results in thev sequence ofoperations outlined. above being Whilst the. cutter is. moving around theperiphery of' the outer pocket the. in-feed movement'r is continuously advancing the impeller towards the cutter. The peripheral and in-feed movements are stopped when the` outer pocket has been completely machined. The impeller I0 is. then. removed from the machine shown inFigure '7 and mounted` in the machine shown in Figure 8 so that theA inner pocket may now be produced.

The Huid system of the machine shown in Figure 8 is identical with. that shown in Figure '7 and in these two figures like parts are indicated' by Referring to Figure 8: the impeller Ill is mounted upon a table II which is carried by a workhead generally indicated at I2. As stated above the inner pocket is formed by moving the cutter-relatively to the impeller in a circumferential and radial direction. To allow Viorthis the table II is rotatable about aXis f in order toA provide the` circumferential movement referred to, and they head I2 is reciprocatable along slide 53 in order to provide for the radial movement. The circumferential movement s performed vby motor` I4 oscillating table II and like numerals.

Athe reciprocating motion of thesheadVL I2 by mo- -to'r I 6.

Carried by workhead I2 is a laminar pattern 54 which is reciprocated Withthe impellen Co'- operating withY the pattern is a tracer pin 55 which is associated with the cutter I3. Theoutline of the pattern has the shape shown inFigureV 6 and corresponds with the cross-sectional shape in a radial plane of the inner pocket which is to be produced by the machining operation. The tracer pin 55 is maintained stationary Whilst the pattern 54 is moved relatively thereto, the arrangement being that upon the pattern engaging the tracer pin the oscillating or recip- -rocating movement of the impeller is stopped.

The operation of the machine is substantially the same as already described with reference to Figure '7 and accordingly this will now only be briefly outlined. Y

Let it be assumed that the cutter I3 is in en gagement with the impeller at the point h (see Figure 5) and is about to trace around the triangular path h, i, h. The disposition of the various parts of the mechanism at the commencement of this movement is shown in Figure 8. Pressure fluid from the pump I8 passes to the right hand end of the motor I4 and to the left-hand end of the motor I6, the former motor being at the end of its clockwise stroke as seen from above in Figure 8 and the latter motor being prevented from feeding the Work further to the right in Figure 8 by the tracer 55 engaging the pattern 54. The pressure rises in both mo' tors and the pilot valves 36 and 40 are opened so that pressure iiuid passes to the actuators 21 and 24 so as to displace the valve 23 to its other setting and maintain the valve 22 in the setting shown. The valve 23 then admits pressure fluid to the left-hand end of the motor I4 which oscillates the Work counterclockwise as seen from above about the axis f which is coincident with that of the cutter. When the motor I4 has completed its stroke, the iiuid pressure in both motors again rises to the predetermined value at which it opens the pilot valves 36 and 38. The valve 36 admits pressure iiuid to the actuator 21 to maintain the valve 23 in its existing setting, i. e., the opposite to that shown, While the valve 38 admits pressure iiuid to the actuator 25 which adjusts the valve 22 to its opposite setting to that shown. Pressure fluid from pump I8 passes by pipe 28, assembly I 9, and pipe 29 to motor u lto pilot valves 35, 38 are opened so that pres- Actusure fluid may pass to actuators 25, 26. ator 26 displaces valve 23 and permits the pressure fluid to pass from pipe 49 to pipe 3I. The

ltransmission of pressure fluid to actuator 25 has no effect and accordingly motor I6 will maintain a pressure tending to move the head I2 towards the left in the direction of arrowg. This however is prevented by the engagement of the tracer pin with the pattern 54. Motor I4 therefore rotates table I I about axis f so that the cutter moves vcircumferentially :over the work until point z'- is reached (see Figure 5) when the motor I4 reaches the end of its stroke which prevents further movement.v 'I'he pressure increase which results, on being transmitted to pilot valves 35, 40, opens these and 5 permits the pressure iiuid to pass to thev actuators 24, 26. Actuator 24 places valve 22 so that pressure uid from pump I8 passes by pipe 30 4to motor I6'. The pressure iluid passing tothe At the same time pressure uid passes by actuator 25 produces no change in, the `setting of the valve 23.

When the motor I6 has completed this reciprocating movement the cutter will have returned to its starting point h at which point the tracer pin will engage with the pattern and prevent further movement in the direction g. The increase in fluid pressure which results from this engagement is transmitted to the pilot valves and actuators so that the work is again oscillated about the axis f with which the cutter is now coincident. The sequence of operations outlined above is then repeated.

As the cutter is advanced into the work the tracer 55 moves into the pattern 54 and the extent to which the work is moved to the right in Figure 8 by the motor I6 is reduced. The travel of the cutter to the left relatively to the work is thus reduced so that it will not reach the axis f and will' move along a path concentric with the axis f when the work is next oscillated about the axis f. The cutter will thus produce the surface lc'shown in Figure 6 of which the shape in a radial plane is determined by that of the pattern.

It will be clear from the above descriptions of the operation of Figures 7 and 8 that during the rst machining operation the movement Of the impeller by motors I4 and I6 is of constant magnitude throughout the operation. It is thus possible to provide fixedV limits for each such movement. As already indicated the limit of the movement is reached when the piston of the motors I4 and I6 have completed their travel. During the second machining operation the movement of the impeller in the radial direction constantly diminishes during the machining operation, being greatest at the commencement of the operation and a minimum at the end thereof. Accordingly it is impossible to p 'rojvide the xed stops in the radial direction as in Figure 7 and instead the pattern 54 and tracer 55 are provided.

To enable manual adjustment of the impeller relativel to the cutter to take Vplace Whilst mounting the impeller on the machine, hand valve 48 is provided. This valve is capable of placing pipe 29 in communication with pipe 30. When this is done in the machine shown in Figure 7 the two sides of the piston of motor I'4 are placed in communication so that manual oscillation of the table II can take place since iluid displaced from one side of the piston passes to the other. At the same time the head I2 can be freely swung by hand since the one side of piston of motor I5 communicates by pipe 3I or 32 and valve assembly 20 with one of the discharge pipes leading from the assembly, Whilst the other side of the piston communicates by pipe 3I or 32, valve assembly 2U, pipes 49, 28, valve assembly I9, pipesA 29 0r 30, with one of the discharge pipes leading from the assembly I9. A similar state of affairs is produced when the valve 48 of Figure 8 is set in the open position.

It is preferred that the machines described with reference to Figures '7 and 8 be operated by oil under pressure since this enables heavier loads to be transmitted. Nevertheless, the machines may be operated by compressed air.

In certain circumstances it may be found desirable to eliminate springs 46 from the pilot valves of the machines described with reference to Figures 7 and 8 since unless these are of considerable length they have a tendency to break.

-With this end in view pump I 8 may pass pressure fluid to a valve 50 which permits high pressure fluid to pass by pipes 28, 49 to the valve assemblies I9, 20. This valve incorporates a reducing valve which ensures that the pump also delivers low pressure iiuid by a pipe line 52 which leads to the upper surface of the plunger 45 of each of the pilot valves. Build up of pressure above the plungers is prevented by leakage past them to the drain 56. The plunger of each pilot valve is maintained in its closed position by the low pressure fluid which constantly acts on the upper surface of the plunger. The liquid displaced by the upward movement of one plunger is displaced into the space left by the downward movement of another plunger which occurs concurrently once the cycle of operations has been initiated. Thus the valve 22 as shown in Figure 8 connects the pipes 28 and 34, so that high pressure acts underneath the plunger of the valve 36 to raise it, and connects the pipe 33 to a drain 51 so that the low pressure on top of the valve 35 holds it down. When the valve 22 is moved to its opposite setting, it connects the pipes 28 and 33 and the pipes 51 and 34. High pressure thus acts vunder the plunger in the valve 35 to raise it while the plunger in the valve 36 has no pressure under it so that the low pressure above it moves it down.

We claim: Y

1. A machine tool comprising a cutter, a worksupport, a reversible fluid motor arranged to move the work-support relatively to the cutter in one direction, a second reversible fluid motor arranged to move the work-support relatively to the cutter in a direction transverse to said first direction, means limiting the extent of movement of the work-support relatively to the cutter in each direction, a source of uid under pressure, a separate control valve controlling the admission of pressure uid from said source to each motor and operable to reverse the direction of action of that motor, and four pilot valves, one for each direction of action of each motor, each subjected to the fluid pressure within the associated motor actuating that motor in the associated direction and, in opposition thereto, by a constant load in excess of the pressure in the associated motor when that motor is moving the work-support relatively to the cutter in the associated direction and each arranged to be actuated by a rise in the pressure to which it is subjected above said constant load to actuate the control valve associated with the other motor to reverse the direction of action of that motor.

2. A machine tool comprising a cutter, a worksupport movable relatively to the cutter in two directions transverse to one another, two reversible uid motors operatively connected to the work-support to move it relatively to the cutter in the two said directions respectively, means limiting the extent of movement in each direction, a source of fluid under pressure, a separate control valve for each motor which in one setting admits pressure iiuid from said source to its motor to cause it to act in one direction and in another setting admits fluid to that motor to cause it to act in the reverse direction, two fluid pressure actuators for each control valve one of which is connected to the valve to move it to one setting and the other is connected to the valve to move it to the other setting, a separate pilot valve for each actuator operable to admit pressure fluid to its actuator and thereby bring an associated motor into action in an associated direction, a separate connection from each control valve to the two pilot valves associated with the other control valve by which one or the other pilot valve, depending on the setting of the control valve, is subjected to fluid pressure from the said source tending to open it and means applying a constant load to each pilot valve, tending to close it, in excess of the fluid pressure in either motor when it is moving the Worksupport relatively to the cutter.

3. A machine tool according to claim 2, wherein the means applying a constant load to each pilot valve subjects that valve to iiuid pressure less than that required to actuate either motor.

4. A machine tool according to claim 2, wherein the means applying a constant load to each pilot valve comprises a connection from that valve to the source of fluid and a reducing valve in that connection.

5. A machine tool comprising a cutter, a Worksupport mounted to move relatively to the cutter in two directions transverse to one another, two double-acting piston and cylinder motors which are connected to the work-support to move it relatively to the cutter in said two directions respectively, a source of fluid under pressure, a separate control valve for each motor which, in one setting, admits fluid from the source to one endof its motor cylinder and, in a second setting admits fluid from the source to the other end of the motor cylinder to reverse its direction of action and which are arranged for adjustment from one setting to the other by fluid pressure, four pilot valves associated one with each setting of each control valve and arranged each, when open, to admit fluid from the source to the associated control valve to move it to the associated setting, connections from each pair of pilot valves associated with one control valve to the opposite ends, respectively, of the motor cylinder associated with the other control valve by which each pilot valve is subjected to the iiuid pressure in one end of one motor cylinder in the direction to open the valve, and means applying a, constant load to each pilot valve to close it which load is greater than the pressure required to actuate either motor but is less than the pressure built up in either motor when it reaches the end of its stroke.

6. A machine tool according to claim 5, comprising a pattern and a follower cooperating with the pattern, which pattern and follower are mounted for relative movement parallel to one of the directions of movement of the work-support 'relatively to the cutter and to move relatively to one another to an extent corresponding to the movement of the work-support relatively to the cutter in said direction.

JOHN THOMAS BOLAS. RAYMOND WILLIAM EDWARDS. 

